Magnetic recording media are widely used as audio tapes, video tapes, magnetic tapes for computers and the like.
The magnetic recording medium basically comprises a non-magnetic support and magnetic layers formed thereon.
Recently, for magnetic recording media such as audio tapes, video tapes and magnetic tapes for computers, there have been strong demands for higher density recording, for example, by shortening recording wavelengths together with requirements to further improve electromagnetic characteristics.
In response to such demands, the coercive force of fine ferromagnetic powders used has been improved. For example, fine ferromagnetic powders having a coercive force of 500 Oe or more have been used. However, even if the fine ferromagnetic powders having high coercive force are used, there remains the problem that improvements in electromagnetic characteristics are insufficient. In particular, improvements in reproduced outputs of short wavelengths are inadequate.
Usually, various signals ranging from a short wavelength to a long wavelength are recorded on the magnetic recording medium. These signals are not uniformly recorded in the magnetic layer, but are recorded in different portions thereof depending on their wavelength.
Specifically, the signals on the short wavelength side (.lambda..ltoreq.0.6 .mu.m) have a tendency to be recorded in portions relatively near to the surface of the magnetic layer, and the signals on the long wavelength side (.lambda..gtoreq.about 2 .mu.m) are more likely to be recorded at greater depth portions (portions near to the non-magnetic support) of the magnetic layer.
On the other hand, when attention is given to the relationship between the reproduced output of a wavelength of a recorded signal and the filling rate of the fine ferromagnetic powder of the magnetic layer, an improvement in reproduced output is not necessarily proportional to an increase in filling rate. The required filling rate of the fine ferromagnetic powder varies depending on the characteristics of the wavelength of the recorded signal.
Namely, to record the signals on the short wavelength side, the filling rate of the fine ferromagnetic powder contained in the magnetic layer (the weight of the fine ferromagnetic powder per unit area of the magnetic layer, unit: g/cm.sup.2) is required to be relatively high, whereas to record the signals on the long wavelength side, the filling rate of the fine ferromagnetic powder is not required to be as high as with the recording of the signals of the short wavelength side.
As methods for improving the electromagnetic characteristics of the magnetic recording media, the methods of increasing the filling rate of the fine ferromagnetic powders through the entire thickness of the magnetic layer are generally used.
As described above, however, the signals recorded on the magnetic recording medium are recorded in different depth portions of the magnetic layer and the filling rate of the fine ferromagnetic powder required for the recording varies depending on the wavelength characteristics of the recorded signal. For this reason, if the fine ferromagnetic powder is uniformly filled through the entire thickness of the magnetic layer at a filling rate more suitable for long wavelength recording, the magnetic layer will be insufficient for short wavelength recording. There is therefore the problem that the reproduced output of the short wavelength signal is not sufficiently improved. On the other hand, if a high filling rate more suitable for short wavelength recording is selected, not only can no improvement be expected in most cases in the reproduced output of the long wavelength signal (which reaches the saturated state at a low filling rate), but also the flexibility of the magnetic layer is reduced, which results in adverse effects on the running properties.
A magnetic recording medium has been proposed having a non-magnetic support and a magnetic layer formed thereon with the magnetic layer containing a fine ferromagnetic powder having a coercive force of 500 Oe or more dispersed in a binder, and in which R.sup.1 /R.sup.2 is 1.1 or more, wherein R.sup.1 represents the average filling rate of the fine ferromagnetic powder within the range from the surface of the magnetic layer to a depth of 1 .mu.m, and R.sup.2 represents the average filling rate of the fine ferromagnetic powder within the range from the magnetic layer face in contact with the non-magnetic support to 1 .mu.m therefrom (JP-A-62-31018, the term "JP-A" as used herein means an "unexamined published Japanese patent application").
However, an object of this proposed magnetic recording medium was to improve the sensitivity of the short wavelength signals by increasing the average density of the ferromagnetic powder in the layer near to the surface (within 1 .mu.m). This proposed magnetic recording medium is actually effective. However, the ferromagnetic powder is not sufficiently suitable for the short wavelength signals, and the binder used is not satisfactory in dispersibility. Consequently, satisfactory C/N can not be obtained. Further, the binder contains an excess of isocyanate, so that migration to the surface of the magnetic layer takes place in the coating step. Also, when the amount of the binder contained in the upper layer of a plural magnetic layer arrangement is reduced to increase the filling rate, a distribution of the binder is produced in the upper layer in order to reduce the filling rate at the surface, which results in inadequate short wavelength recording (.lambda..ltoreq.0.6 .mu.m). If attempts are made to increase the filling rate by sputtering the surface of the single magnetic layer with electron beams (EB) to solve this problem, the excess isocyanate contained in the surface of the magnetic layer are removed. However, a lubricating agent is also liable to be lost at the same time, whereby the problem of deterioration of the running properties tends to occur due to the loss of lubricating agent.